1. Field of the Invention
This invention relates to a vibration wave driven motor which produces a friction drive of movable member with traveling vibration wave.
2. Related Background Art
Suggestions regarding the structure of the vibration wave driven motor which utilizes flex vibration of piezo-electric element is revealed in the Japanese Laid-Open Patent Application No. 63-73887 and others.
Said suggestions offer the following motors:
1) a small-sized and compact vibration wave driven motor; PA0 2) a vibration wave driven motor with excellent heat radiating effects; PA0 3) a vibration wave driven motor with high operating efficiency; and, PA0 4) a vibration wave driven motor with easy to adjust pressurizing force.
Since the driving principle of the vibration wave driven motor is publicly known, detailed explanations are omitted but brief explanations are given as follows.
On the vibration wave driven motor, a piezo-electric element used as an electro-mechanical energy conversion element is bonded with adhesive to one side of a metallic resilient member formed, for example, into a ring shape. By applying AC voltage with respectively differing phases to two piezo electric element groups for driving, formed on said piezo-electric element, two types of standing waves are excited on the resilient member. By composing these standing waves, traveling vibration waves, which are flex vibrations, are generated.
On the other hand, against the other side of the resilient member, a member part in the form, for example, of a ring is press-contact by means of pressurizing with a spring, etc. This member part is moved by friction of traveling waves generated on the resilient member or the resilient member itself is moved.
Explanations are made as follows regarding a conventional example with the aid of FIG. 5.
2 is a motor cover and 1 is a motor case. A resilient member 3, for example, structured with stainless steel is attached with a piezo-electric element 4 and these items configure a stator 5 (vibration member). The stator structure permits conduction and dispersion of the heat resulting from motor temperature rise due to heating of the stator 5 to the case 1 by way of the cover 2.
8 is a rotor structured by fitting a slider 7 to a ring 6. This rotor 8 is pushed against the stator 5 via a rubber member 9 by means of the pressure force of a belleville spring 10. The rotor 8 is able to rotate integrally with a shaft 11. Since the pressure force is maintained by a snap ring 13 after selecting and properly adjusting the thickness of a shim 12, this pressure can be very easily adjusted. The shaft 11 is supported by bearings 14 and 15 mounted on the case 1 and the cover 2 respectively so as to allow free rotation.
FIG. 6 is the drawing of a conventional example that shows the installation status of an encoder device on a vibration wave driven motor.
35 is an encoder fixed on an encoder mounting plate 40 with a screw 41. The encoder device is connected to the shaft 11 of the vibration wave driven motor at a step section 11-b by a coupling 42.
On the above-mentioned conventional example, however, as the shaft 11 is supported by two sets of bearings 14 and 15 mounted on the bearing engagement sections formed on the case 1 and the cover 2 that are separate member parts from each other, the following defects occur.
1) Two bearing sections are configured as a separate member and due to the misalignment of the case 1 and the cover 2, the coaxiality at the bearing sections are increased. Therefore, tumbling of the rotation center of the shaft 11 supported by these bearing sections is caused.
Consequently, parallelism of the opposing surfaces between the rotor 8 fixed on the shaft 11 and the stator 5 fixed on the cover 2 becomes greater in value. To eliminate this defect, on the conventional example said parallelism errors tend to be absorbed by utilizing elasticity obtainable from the support structure of both stator 5 thin section 3-a and rotor 8. However, this attempt is not perfectly successful because it produces contact or wear on one side. Also, when "sharp ramp-up or ramp-down" which is a feature of the vibration wave driven motor occurs, a delay of deflection follow-up of the elastic structure of the rotor 8 and the stator 5 is observed. Further, the reaction force of deflection follow-up tends to create undesired stimulation resulting in adverse effects to the motor and degrading operation efficiency.
2) When perpendicularity is required between the shaft 11 and the mounting surface 2-a of the motor device (for instance, perpendicularity of less than 10 .mu.m), the vibration wave driven motor can be operated by the direct coupling drive method which permits the motor to most effectively demonstrate its other operating feature of generating of high torque at low speed. In many applications of this direct coupling drive method, however, precise perpendicularity of the shaft against the driven section is required. On the other hand in the conventional example, it is difficult to improve the perpendicularity, as mentioned above, between the shaft 11 and the mounting surface 2-a. Consequently, such a defect makes it impossible to utilize this feature of the vibration wave driven motor.
3) As is evident from the fact that 70% of servo motors have encoders inside the motor, the need for such a built-in encoder is quite high. More particularly, when an encoder with high precision and excellent resolution is built in, judging from the characteristics of the encoder, an extremely high-level of positioning between the rotor in the encoder and stator detection portion and coaxiality or vibration accuracy with shaft 11 to which the rotor in the encoder is connected are required. As explained in the above section 2), accuracy improvement is apparently very difficult on the conventional example.
Further, at the structure which the encoder device 35 is connected by the coupling 42, the motor device including the encoder device 35 becomes quite large and makes it difficult to reduce the structural size.
As shown in FIG. 6, since the encoder device 35 is connected by the coupling 42, position detecting errors happen because of alignment errors by the coupling 42, and twisting errors and twisting vibration due to torsion rigidity lower than the shaft 11.